Piezoelectric relay devices are recognized to provide a means for either initiating or interrupting current flow to a load device. A known piezoceramic type relay device for this purpose is disclosed in U.S. Pat. Nos. 4,670,682 and 4,689,517 both assigned to the assignee of the present invention. The relay device includes a piezoceramic bender member formed by at least two planar prepoled piezoceramic plate elements secured in opposed parallel relationship sandwich fashion on opposite sides of at least one central conductive surface and having outer conductive surfaces that are insulated from each other and the central conductive surface by the respective intervening piezoceramic plate element thicknesses. Movable contact associated with the movable bender coact with fixed contacts disposed thereby to either complete or interrupt an electrical circuit providing current flow from a power source to the load device. A representative form of this type relay device as disclosed in the above mentioned prior art patents employs a piezoceramic bender member which is selectively prepoled with clamping means secured at non-poled portions adjacent to and mechanically supporting the selectively prepoled bender member in a cantilever manner for operating pairs of coacting electrical contact means and with the non-poled portions being mechanically unstrained and electrically neutral. The bender member is made to operate either side of a center position normally assumed by the bender member in an unenergized position to thereby enable different modes of operation. In one mode of operation, the relay device can simply serve as an on-off switch wherein one pair of coacting switch contacts either makes or breaks the electrical circuit with respect to the load device. In a different mode of operation, however, a pair of coacting switch contacts is provided on each side of the bender member to enable selective energization of multiple load devices. Both modes of operation with the prior art "bimorph" type bender switching devices are further said to be conducted in a similar manner wherein the DC energization potential used to actuate deflection of the bender member has the same polarity as the polarity of the prepoling potential used to prepolarize the prepoled piezoceramic plate element. The depolarization avoided by operating the relay devices in this manner provides dipole enhancement enabling relatively long term operation with load devices employing load voltages as high as 5000 volts and corresponding currents as high as hundreds of amperes.
In both above defined modes of operation, such piezoceramic relay devices have been recognized to afford major operational and structural advantages over either electromagnetic (EM) relays or semiconductor devices when employed in power switching applications. These advantages are reported in U.S. Pat. No. 4,658,154, also assigned to the present assignee, which further includes disclosure of piezoceramic relay switching circuits providing control of single and double load apparatus. The EM relays still widely employed for this purpose provide an interface between, for example, an electronic control circuit and a load circuit wherein the former handles the low power control signals for selectively energizing the relay coil to appropriately position the relay contacts coacting in the power circuit to switch relatively higher levels of power. When such relay contacts are closed, load current is conveyed, with virtually no losses, and when they are parted, load current is interrupted with the certainty only an air gap can provide. Over the years improvements in EM relays have resulted in increased efficiency and reduced physical size. That is, such relays can be actuated with control signals of rather low energy content to switch reasonably high levels of load current. For example, EM relays are available which can be actuated with a one watt control signal to switch several kilowatts of power at 115 or 230 volts AC. As a consequence, EM relays can be operated with signals generated by solid state control circuitry. On the other hand, the drawbacks associated with EM relays employed for controlling current flow in load circuits responsive to control signals still remains substantial. While current EM relays have been miniaturized as compared to earlier designs of the relays, their actuating power requirements are still quite large in contrast to, for example, state of the art solid state power switches. The current EM relays are still relatively complex and expensive to manufacture, for example, their coils typically require a multitude of turns of very fine wire. The coil resistance, consumes some power which must be provided by a reasonably stiff power supply. When, for example, EM relays are utilized in home appliance controls, relay operating power must be derived from a 115 or 230 volt AC utility source. The requisite power supply, particularly when an EM relay is operatively associated with a solid state control circuit, requires a transformer, electrolytic capacitors, regulators and protection to insure a reliable source of relay actuating current. Such power supplies are both costly and constitute a significant source of power dissipation. Moreover, in certain applications where high ambient magnetic fields are present, such as in motor starter applications, EM relays must be specially shielded to discourage spurious operation. The drawbacks associated with employment of EM relays in power switching circuitry has thereby resulted in a trend toward utilizing solid state switches, such as SCRs, Triacs, Thyristors, MOSFETs, IGTs and the like as the power switching output device. While such solid state switches are becoming relatively inexpensive and may be smaller in physical size than comparably rated EM relays, they do present a rather significant "on" resistance, which, at high current levels, results in considerable power dissipation. Thus, semiconductor power switches being utilized in high current applications must be properly heat-sinked for protection against thermally induced damage, and, as a consequence, with their heat-sinks can take up more physical space than do their EM relay counterparts. Moreover, solid state power switches must be protected against possible damage in spurious operation as a result of transients, electrostatic discharges (ESD) and electromagnetic interference (EMI). All of these protective measures represent an additional expense. In that such solid state power switches do not impose an air gap to restrain the flow of current in their "off" condition and because of their "on" condition failure mode, Underwriters Laboratory has disapproved of their application in numerous domestic appliances. Such disapproval has only been overcome in part with a combination of the solid state switches and the EM relays in some domestic appliances so as to provide the required air gap.
All of the foregoing major disadvantages found with employment of either EM relays or semiconductor switches as the power switching output device has prompted renewed interest in piezoelectric relays, including piezoceramic relay devices. Recent improvements in piezoceramic materials have enhanced their electromechanical efficiency for these relay applications. Piezoceramic drive elements may be fabricated from a number of different polycrystalline ceramic materials such as barium titanate, lead zirconate titanate, lead metaniobate and the like which are precast and fired into a desired shape such as rectangular-shaped ceramic plates. The piezoceramic relay devices require very low actuating current, dissipate minimal power to maintain an actuated state, and draw no current while in their quiescent or unenergized state. The electrical characteristics of the piezoceramic drive elements are basically capacitive in nature, and thus are essentially immune to ambient electromagentic fields. Such piezoceramic relay devices can be designed in smaller physical size than comparably rated EM relays. Since piezoceramic relay devices utilize switch contacts, contact separation introduces the air gap in the load circuit as required for UL approval in domestic appliance applications. Closure of these relay contacts provides a current path of negligible resistance, and thus unlike solid state power switches, introduces essentially no loss in the load circuit. Since additional structural and operational advantages for such improved piezoceramic relay devices can be found in the aforementioned prior art U.S. Pat. Nos. 4,670,682 and 4,689,517, both disclosures are hereby specifically incorporated into the present application in their entirety.
The suitability of such piezoceramic relay devices in controlling current flow within a particular apparatus understandably requires still other factors to be considered. Both operational characteristics desired in the apparatus as well as the environmental conditions being encountered have to be satisfied. For particular domestic appliances utilizing at least one resistive heating element for various household purposes to include electric ranges, toaster ovens, electric clothes dryers and electric fry pans, it is further desirable to provide such apparatus both at a low cost as well as enable low cost of operation by a user. Achieving the latter objective is facilitated by operating the domestic appliances directly from the available line voltage power supply with a minimal number and size of components in the control circuitry and while further reducing any susceptibility of the control circuitry to EMI and line transients. Employment of at least one piezoceramic relay device in the control circuitry could theoretically enable current flow to one or more operatively associated resistive heating elements most efficiently with relative immunity to ambient electromagnetic fields. The piezoceramic relay device is further particularly suited for use in combination with low power-drain electronic circuit components to provide the control signals for actuation of the piezoceramic bender member and thereby enable the relay contacts to be either opened or closed. Simplification of the control circuitry is particularly desirable for all of the above illustrated household appliances since it permits more space to be utilized for work tasks and makes it easier to contain the entire control means within the apparatus for a cleaner design appearance.
Understandably, the ability of a piezoceramic relay device and its associated control circuitry to function properly in a relatively high temperature environment represents a still further important consideration. The conventional control means now being employed in electric ranges are required to operate at a minimum 70.degree. C. ambient for extensive time periods. Thus, relay contacts must open and close reliably in this operating environment over the relatively long lifetime demanded for most household appliances. While piezoceramic relay devices have been found capable of long term reliable operation, significant problems are recognized to still exist and which have heretofore only been ameliorated with additional circuit means being employed. Specifically, contact arcing is experienced for different reasons as the relay contacts are opened and closed and which has required additional circuit means to reduce wear and tear at the contact interface. The arcing problem occurring when the contacts are opened is attributed to a rise of reapplied forward potential across the contacts as they open which can be lessened with snubber circuits as proposed in both aforementioned U.S. Pat. Nos. 4,658,154 and 4,670,682 patents. The arcing problem which occurs when these contacts are closed is attributed to mechanical bounce upon closure and this problem is dealt with in a still further commonly assigned U.S. Pat. No. 4,626,698. As therein proposed, novel zero crossing synchronous AC switching circuits are utilized with a piezoceramic relay device including circuit means to initially impress a relatively low voltage energization potential across the piezoceramic bender member to soften its movement and curtail contact bounce after initial contact closure. It is also proposed therein that such circuits be operated to extinguish current flow through the contacts when being opened to help alleviate the former arcing problem. The seriousness of both arcing problems can be appreciated from a still further recommendation appearing in said reference for utilization of specialized contact metals to withstand arc formation whenever the relay contacts are being separated.
Recent legislation in many states now requires domestic appliances to meet minimum energy efficiency standards. For the above illustrated domestic electrical heating appliances such a requirement understandably dictates efficient use of electrical power whenever operating the particular apparatus. To further illustrate the general nature of this problem in connection with conventional electric range operation, one common control means utilizes a temperature sensing element, such as a thermostat, in combination with a programmed microprocessor control unit to effect automated temperature feedback control with respect to the resistive heating elements being employed in both surface heating and oven units. Power is applied to the resistive heating elements responsive to the thermal control means as further based upon the power level setting selected by the operator. The percentage of time power is applied in accordance with the power level setting is customarily termed the duty cycle in the known time-ratio mode of operation. Since the conventional resistive heating elements operate with considerable thermal inertia, however, such thermal control means has proven rather energy inefficient at both high and low power level settings. More energy efficient electronic control means for an electric range are disclosed in a further commonly assigned U.S. Pat. No. 4,443,690, whereby such temperature sensing means can be eliminated. In accordance with the improved control system, the power setting selected by the operator is monitored for utilization with electronic counter means provided in the microprocessor control unit and which is incremented or decremented at a rate that is approximately proportional to the rate of increase or decrease of temperature of the controlled heating element for that power setting. By knowing the operator setting and counting zero crossover points in the microprocessor, the supply power to a particular heating element as determined by emperically determining the thermal losses for different power levels, the effective energy in the heating unit is determined. This determines the increment rate and the count of the counter becomes a measure of the effective energy in the heating unit at any given time. Where the thermal mass of a particular heating element is large, the increment rates can be selected very accurately, regardless of load conditions. A more energy efficient control can be exercised with the improved control system. For example, when a controlled heating element is already operating at a high power setting and the operator selects a lower power setting there will be no power applied to the heating element until needed to maintain the lower power setting. On the other hand, selection of a higher power setting for a heating element being operated at lower power setting results in full power being applied to the heating element in order to decrease the time required to reach the higher power setting. Since the improved control system employs no temperature sensing means or closed loop temperature control circuitry as required in the conventional control means but rather utilizes the same circuitry of the existing microprocessor control unit, it can be further appreciated that incremental cost for its implementation is essentially negligible.
As recognized in the aforementioned U.S. Pat. No. 4,658,154, the operation of a piezoceramic relay to regulate power input to a pair of resistive load devices in a manner precluding simultaneous operation of the respective devices can be carried out with minimum power consumption. Such operational control of the relay device as therein recognized employs high voltage integrated circuitry being powered directly from a conventional utility source such as available 115 volt or 230 volt AC power sources. For such relay control circuitry to efficiently and reliably regulate power input to the individual heating elements now employed in various electric heating apparatus requires that a number and variety of still other important criteria be met. Typical criteria can further be illustrated in connection with the conventional oven control means now being utilized in a household electric range. With the control management employed in the above cited U.S. Pat. No. 4,443,690 patent as well as in other conventional control systems for the electric range, the load current is switched to the individual heating elements by electromechanical means. The switching function is now done either with solid state switches such as high current transistors, triacs, etc. or with electromagnetic relays. As previously indicated, the solid state switching devices require additional heat-sink means and/or fans to remain operational by reason of the typically high current loads being required by the heating elements. Moreover, domestic employment of solid state switching devices necessitates conventional use in combination with an electromagnetic relay to satisfy UL requirements for an air gap interruption of the electrical circuit as previously indicated. The electromagnetic relays now in current use with the aforementioned microprocessor control arrangement have an additional drawback in making the domestic appliance noisy to operate. Since a relay device is actuated several times per minute in a domestic appliance, a distinct audible noise is caused each operating cycle attributable to the impact. Replacing the conventional switching means now employed in a domestic electric heating appliance having multiple resistive heating elements with a piezoceramic relay device thereby affords significant advantages. In the illustrated electric range, a piezoceramic relay is capable of switching these high wattage loads for at least one and up to ten million duty cycles now required over the design life of the appliance. A relatively quiet operation by the device as well as simple control circuitry needed for its operation represent a significant further advantage. More energy efficient operation hence a lower cost of operation by the operator can also be expected for a piezoceramic relay device which can be actuated with microwatts as compared with the watt power requirements for the now employed electromagnetic relay devices. Additionally, the employment of a piezoceramic relay as the power output switching device in this type domestic appliance further enables the controlled apparatus to be operated automatically with simple solid state control means.
The customary circuit relationship for multiple heating elements in most electric heating apparatus is by parallel connection across the power conductors and with the individual heating elements being further connected in series to at least one of the now employed electromagnetic relay devices. It is further common in domestic electric ranges to require that a pair of the switching devices be series connected to each heating element as a safety precaution insuring that each power conductor in the power supply will be interrupted. From these considerations it can be appreciated that replacing the electromagnetic relays with piezoceramic relay devices afford a far more reliable switching arrangement. Simultaneous operation of the respective heating elements due to malfunction of the switching devices can thereby be avoided to a much greater extent. As compared with the electromagnetic relays, the lower actuating power requirements, simpler construction and simpler control circuitry for a piezoceramic relay device as well as its relative immunity to spurious influences effectively enables a fail-safe manner of operation.
It is a principal object of the present invention, therefore, to provide a more energy efficient system for the regulation of electrical power in an electric heating apparatus employing at least one resistive heating element.
It is still another important object of the present invention to provide control means employing at least one piezoceramic relay device to regulate electrical power input in an electric heating apparatus employing at least one resistive heating element in a more fail-safe manner.
A still further important object of the present invention is to provide control means for the regulation of electrical power in an electric heating apparatus employing at least one resistive heating element with improved temperature control means.
Still another important object of the present invention is to provide improved electronic control means for automatic regulation of electrical power in an electric heating apparatus employing at least one resistive heating element.
A still further important object of the present invention is to provide a novel method for regulation of electrical power in an electric heating apparatus employing at least one resistive heating element.
Still a further important object of the present invention is to provide a method of operating piezoceramic relay means to more efficiently regulate electrical power input to at least one resistive heating element in an electric heating apparatus.
Still another important object of the present invention is to provide a more efficient method to automatically regulate electrical power input in an electric heating apparatus employing at least one heating element.
Another important object of the present invention is to provide a more efficient electric heating apparatus utilizing novel control mean to more reliably regulate electrical power input to at least one resistive heating element.
Another important object of the present invention is to provide control means regulating electrical power input in an electric heating apparatus utilizing a plurality of resistive heating elements in a manner avoiding unintended simultaneous operation of the respective heating elements.
Still another important object of the present invention is to provide an electric heating apparatus employing simpler and lower cost control means to regulate electrical power input to at least one resistive heating element with piezoceramic relay means.
A still further important object of the present invention is to provide an electric heating apparatus employing improved electronic control means to automatically regulate electrical power input to one or more resistive heating elements.
A still further important object of the present invention is to provide an electric range utilizing novel control means to regulate electrical power to a plurality of resistive heating elements.
Another important object of the present invention is to provide a household cooking appliance employing novel control means to automatically regulate electrical power input to at least one resistive heating element in a more energy efficient manner.
These and still further objects of the present invention will become apparent upon considering the following detailed description for the present invention.